Medical implants are used for innumerable medical purposes, including the reinforcement of recently re-enlarged lumens, the replacement of ruptured vessels, and the treatment of disease such as vascular disease by local pharmacotherapy, i.e., delivering therapeutic drug doses to target tissues while minimizing systemic side effects. Such localized delivery of therapeutic agents has been proposed or achieved using medical implants which both support a lumen within a patient's body and place appropriate coatings containing absorbable therapeutic agents at the implant location. Examples of such medical devices include stents, stent grafts, vascular grafts, catheters, guide wires, balloons, filters (e.g., vena cava filters), intraluminal paving systems, implants and other devices used in connection with drug-loaded polymer coatings. Such medical devices are implanted or otherwise utilized in body lumina and organs such as the coronary vasculature, esophagus, trachea, colon, biliary tract, urinary tract, prostate, brain, and the like.
The delivery of stents is a specific example of a medical procedure that may involve the deployment of coated implants. Stents are tube-like medical devices designed to be placed within the inner walls of a lumen within the body of a patient. The tube walls of the stents are typically patterned, leaving openings, with the material of the stent forming a scaffold for the lumen wall. Stents are made, for example, of stainless steel, Tantalum, Platinum or Nitinol alloys. The stents are maneuvered to a desired location within a lumen of the patient's body, and then typically expanded to provide internal support for the lumen. Stents may be self-expanding or, alternatively, may require external forces to expand them, such as by inflating a balloon attached to the distal end of the stent delivery catheter.
The mechanical process of applying a coating onto a stent may be accomplished in a variety of ways, including, for example, spraying the coating substance onto the stent, so-called spin-dipping, i.e., dipping a spinning stent into a coating solution to achieve the desired coating, and electrohydrodynamic fluid deposition, i.e., applying an electrical potential difference between a coating fluid and a target to cause the coating fluid to be discharged from the dispensing point and drawn toward the target. In these prior stent coating systems, the stents typically are coated on all surfaces. For example, with a coating spray application system, the relatively open patterned structure of the stent permits a coating spray to pass through the open areas and coat the inner surfaces of the stent. Similarly, with a spin-dipping stent coating system, all the surfaces of the stent, interior and exterior, are exposed to the coating fluid upon immersion into the coating bath.
In the typical stent deployment, the outside surface of the stent contacts the vessel wall. The inside surface of the stent is exposed to the fluid, e.g., blood, passing through the lumen.